Lithographic printing, so-called offset printing, is the mainstream in the printing on paper and is widely used. Printing plates for use in the offset printing have been heretofore produced as follows: a printing original is temporarily output on paper or the like, then the original is photographed to prepare a comprehensive film, and through the comprehensive film a photosensitive original plate is exposed and developed.
However, with recent digitization of information and increase of laser power, a process wherein drawing is directly made on an original plate by laser scanning to prepare a printing plate without using the comprehensive film, a so-called CTP (Computer To Plate) process, has been put into practical use in the production of printing plates.
As the plate for the CTP process practically used at present, there is a photopolymer type plate using photo-reaction due to a visible light of about 500 nm. This plate, however, has problems that the plate needs development, has poor resolution and cannot be handled in a bright room.
In order to solve such problems, a plate using thermal reaction due to a light of near infrared region is disclosed in Japanese Patent Laid-open Publication No. 20629 (1995), and this plate has been already on the market.
Although this plate can be certainly handled in a bright room and has excellent resolution, it still needs a process of development.
In Japanese Patent Laid-Open Publication No. 282142 (1996), a plate having a non-image area formed from a hydrophilic swelling layer is disclosed. In this plate, a hydrophilic swelling layer is formed, and a photosensitive material is absorbed by the hydrophilic swelling layer to allow the layer to have photosensitivity. In the image area, the photosensitive material in the hydrophilic swelling layer undergoes reaction by exposure to light and the image area loses hydrophilicity, but the ink receptivity is not sufficient. On the other hand, the photosensitive material remains in the non-image area, and after the exposure, rinsing to remove the photosensitive material from the non-image area is necessary.
As a plate which does not need a process of development, a plate comprising a substrate having provided thereon an inorganic light absorbing layer comprising titanium or titanium oxide and an ink-repellent layer made of a silicone resin in this order is disclosed in Japanese Patent Laid-Open Publication No. 314934(1995), and also this plate has been already on the market. In this plate the silicone resin layer repels an ink and becomes a non-image area, while an image area is formed by irradiation with a near infrared light. The silicone resin layer is removed by the irradiation with a light to expose the ink-receptive substrate surface outside. In order to completely remove the silicone resin layer, wiping-off is necessary, and if the wiping-off of the silicone resin is insufficient, an ink receptivity on the irradiated area is not sufficient to cause defects in the image area, and printing is not made satisfactorily.
In Japanese Patent Laid-Open Publication No. 199064 (1994), there is disclosed a plate consisting of a substrate, a light absorbing layer provided thereon in which carbon black is dispersed in nitro cellulose, and a hydrophilic layer or an ink-repellent layer laminated on the light absorbing layer. In this plate, the light absorbing layer is thermally decomposed upon irradiation with a light, and the light absorbing layer and the hydrophilic layer or the ink-repellent layer provided thereon are removed to expose the ink-receptive substrate surface outside. That is, imaging area is made by so-called ablation. This plate can be handled even in a bright room and does not need operations of development and wiping-off. However, great energy is necessary to remove the light absorbing layer and the hydrophilic layer or the ink-repellent layer provided thereon, and a long period of time is necessary for the exposure. Moreover, there is a problem that a part of the thus removed light absorbing layer, hydrophilic layer or ink-repellent layer provided Whereon and their decomposition products accumulate on the unexposed hydrophilic layer or ink-repellent layer around the exposed area, to cause deterioration of qualities such as adhesion of an ink.
As a no ablation plate, technique such that a photosensitive layer comprising hydroxyethyl cellulose, a phenol resin and a photo-radical generator is cured by irradiation with a light and thereby becomes lipophilic is disclosed in U.S. Pat. No. 3,793,033. In this plate, however, the balance between the hydrophilicity and the lipophilicity after the irradiation with a light is bad and fine printing cannot be made.
In Japanese Patent Laid-Open Publication No. 52932 (1985), there is disclosed a plate such that a surface of a non-water-absorbing resin layer is sulfonated to make the surface hydrophilic and the sulfonated surface layer is removed by irradiation with a light to make the layer lipophilic. In this case, ablation is used, but only the surface layer is ablated, so that debris scarcely occurs. From this viewpoint, this plate has been improved, but the plate is unfavorable because the hydrophilicity is insufficient, scumming tends to occur, and the sulfonation process is complicated and dangerous.
In Japanese Patent Laid-Open Publications No. 127683 (1997) and No. 171249 (1997), there are disclosed plates comprising a hydrophilic support and a photosensitive layer made from an aqueous dispersion of thermoplastic polymer particles which are softened and/or melted by exposure to light and changed to ink-receptive. The unexposed area of the photosensitive layer of these plates is soluble in water and is easily removable, so that an exclusive developing machine is unnecessary and development can be carried out on a printing press using a fountain solution. That is, these plates have been put into practical use as those for on-press development system. In the on-press development, however, there reside problems that the fountain solution and the ink are contaminated and strict control of the moisture of the plate is required.
As a plate which needs neither wet development nor on-press development, a plate having a hydrophilic resin layer containing hydrophobic thermoplastic polymer particles which are independent from one another and in contact with one another, said hydrophobic polymer particles being melted by heat to change hydrophilicity of the resin layer, is disclosed in U.S. Pat. No. 3,476,937. However, when drawing is made on the plate by irradiation with a light, the sensitivity is low, and the hydrophilic resin layer has low mechanical strength and durability of the plate is poor. Moreover, if the amount of the hydrophobic thermoplastic polymer is increased to improve the ink receptivity, scumming is liable to take place.
In Japanese Patent Laid-Open Publication No. 1850 (1995), there is disclosed a plate having a photosensitive layer comprising a hydrophilic resin and microcapsules contained in the resin, said microcapsules containing a lipophilic material that is reactive to hydrophilic groups of the hydrophilic resin, and there is also disclosed technique of irradiating the plate with a light to rupture the microcapsules and to change the hydrophilic resin to lipophilic. In this technique, however, to increase resolution or to prevent scumming, the diameters of the microcapsules must be decreased, and it is very difficult to produce the microcapsules. In the printing using thermal head, the microcapsules are ruptured relatively easily by heat and pressure, but in the printing using irradiation with a light, the microcapsules are not ruptured uniformly and the resolution is bad.
Further, there has been proposed a process wherein a substrate with a light absorbing layer containing a resin or the like formed thereon is brought into close contact with another substrate and irradiated with a light to transfer the light absorbing layer to another substrate using heat generated by the irradiation. However, there are various problems in this process, for example, it is difficult to bring the substrates into close contact with each other because dust or the like attaches to the substrates, great energy is necessary for the transferring, and the transferred light absorbing layer has low mechanical strength and comes away during the printing.
As described above, the conventional CTP printing plates have various problems, and therefore, development of CTP printing plates to solve the problems has been desired strongly.
The present invention is intended to solve such problems associated with the prior art as described above, and it is an object of the invention to provide a printing original plate which can be handled in a bright room, does not need operations of development and wiping-off and is excellent in sensitivity, resolution and various printing properties and is used for a CTP printing plate, and to provide a printing plate using the printing original plate and a process for producing the printing plate.